Small increase in likelihood of greater bleeding during menses after COVID-19 vaccination

Key takeaways :

• More COVID-19 vaccinated women reported an increase in total bleeding quantity vs. unvaccinated women.
• By the next menstrual cycle following COVID-19 vaccination, this increase resolved.

Women reported a small increase in the likelihood of greater total bleeding during a menstrual cycle after receiving the COVID-19 vaccine, according to an observational study published in BJOG.

“Women vaccinated for COVID-19 have a slightly higher risk for a heavier period after vaccination; however, changes typically resolve by the first post-vaccination menstrual cycle,” Blair G. Darney, PhD, MPH, researcher in the department of obstetrics and gynecology at Oregon Health & Science University, Portland, and the Population Health Research Center at the National Institute of Public Health, Cuernavaca, Mexico, told Healio. “The findings shouldn’t be cause for concern, but should validate the public’s experiences and provide reassurance that if changes in flow occur with vaccination, they are likely to be small and temporary.”

Researcher analyzed prospectively collected data from 9,555 women, of whom 7,401 were vaccinated against COVID-19 and 2,154 were unvaccinated. Data were from a minimum of four consecutive menstrual cycles for all women — three menses prior to vaccination and at least one following vaccination for vaccinated women — using the digital Natural Cycles app, an app to track fertility used by women not on hormonal contraception. Women reported their daily bleeding quantity as spotting, light, medium or heavy.

The primary outcome was the mean number of heavy bleeding days and changes in bleeding quantity at first COVID-19 vaccination dose, second dose and post-exposure menses compared with unvaccinated women.

Approximately two-thirds of women reported no change in the number of heavy bleeding days regardless of their vaccination status, according to the researchers. In addition, after adjusting for confounding factors, researchers observed no significant differences in the number of heavy bleeding days by vaccination status.

Women who were vaccinated reported an increase in their total bleeding quantity compared with unvaccinated women during the first-dose menses (38.3% vs. 34.8%; P = .027) and the second-dose menses (39.8% vs. 35.5%; P < .001). This translated to approximately 40 additional people per 1,000 women with normal menstrual cycles who reported a greater total bleeding quantity after receiving their first COVID-19 vaccination dose, the researchers wrote. However, this difference resolved in the next cycle after vaccination, according to the researchers.

“Looking forward, we hope to learn more about the biological mechanism of these changes and are also interested in understanding other reported variations, such as missed cycles, unexpected vaginal bleeding and pain,” Darney said. “Additionally, there is more research needed around the effect of COVID-19 disease on the menstrual cycle, since it’s likely that infection itself creates similar changes to those experienced after vaccination.”

For more information:

Blair Darney, PhD, MPH, can be reached at darneyb@ohsu.edu.